Not to hurt anyones feelings or anything but i have the 6.0cc center plug Fred Head onOn my bike I'm using the original slant head and needed to rotate it 90 degrees to get it to fit properly. Definitely won't fit center fire aluminum head without modifying. My slant head goes on the correct way but it's a huge pain to remove the plug and wire that way. So I rotated the head and indexed the spark plug towards the intake.
Oh yeah, i forgot about the engine position with a jackshaft.No feelings hurt. I know it will fit. Alot has to do with the mounting angle. Because I'm running a jackshaft on mine the engine needs tho sit up a little higher on the frame. That's a nice looking bike you have there HP.
Keep er greasy.